The present invention pertains to improvements in the field of fiber reinforced cement-based materials. More particularly, the invention relates to a metal fiber having an optimized geometry for reinforcing cement-based materials.
All cement-based materials are weak in tension. In addition, these materials have a very low strain capacity which places them in a brittle category with other brittle materials such as glass and ceramics. It is well known that concrete and other portland cement-based materials may be reinforced with short, randomly distributed fibers of steel to improve upon their mechanical properties. It is also known that for any improvement in the tensile strength, fiber volume fraction has to exceed a certain critical value.
Beyond matrix cracking, fibers form stress transfer bridges and hold matrix cracks together such that a further crack opening or propagation causes the fibers to undergo pull-out from the matrix. Pull-out processes being energy intensive, steel fiber reinforced concrete exhibits a stable load-deflection behavior in the region beyond matrix-cracking which places these materials in a category of pseudo-plastic or tough materials such as steel and polymers. Thus, while a plain unreinforced matrix fails in a brittle manner at the occurrence of cracking stresses, the ductile fibers in fiber reinforced concrete continue to carry stresses beyond matrix cracking which helps maintaining structural integrity and cohesiveness in the material. Further, if properly designed, fibers undergo pull-out processes and the frictional work needed for pull-out leads to a significantly improved energy absorption capability. Therefore, fiber reinforced concrete exhibits better performance not only under static and quasi-statically applied loads but also under fatigue, impact and impulsive loadings. This energy absorption attribute of fiber reinforced concrete is often termed "toughness".
Concrete is a strain-softening, micro-cracking material. In steel fiber reinforced cement-based composites, fiber bridging action sets in even prior to the occurrence of the perceived matrix macro-cracking. The critical fiber volume fraction or the magnitude of strength improvement at a certain fiber volume fraction, therefore, depends upon the geometry of the fiber. Also dependent upon the geometry is the pull-out resistance of an individual fiber from the cementitious matrix around it, which in turn, governs the shape of the load-deflection plot beyond matrix cracking and the achievable improvement in composite toughness.
An improvement in the strength of the composite at a certain fiber volume fraction or, in other words, a reduction in the required critical fiber volume fraction, is possible by excessively deforming the fiber. However, this may lead to too good a fiber anchorage with the matrix and causes a brittle mode of fracture in the post-matrix cracking region. Toughness reductions in the case of excessively deformed fibers, therefore, can be significant. The other possible way is to increase the number of fibers in the composite by reducing the size of the fibers. This solution is known to cause extreme difficulties in terms of concrete mixing and workability, and uniform fiber dispersion often becomes impossible as the fibers tend to clump together giving a highly non-uniform distribution.
In U.S. Pat. No. 4,585,487, which proposes a concrete-reinforcing fiber having uniform wave shaped corrugations distributed over its entire length, the sole fiber performance characteristics considered for optimization is the fiber pull-out performance. The same also applies in respect of Canadian Patent Nos. 926,146 and 1,023,395, which disclose concrete-reinforcing fibers having a straight central portion with shaped ends. Some fibers have ends which are formed thicker; others have ends which are hooked. All these characteristics are intended to improve anchoring of the fiber in the concrete.
For fibers that are used as a reinforcement distributed randomly in a moldable concrete matrix, the property of interest is the overall composite toughness. The composite toughness, although dependent on the pull-out resistance of fibers, cannot quantitatively be derived from the results of an ideal fiber pull-out test where the fiber is aligned with respect to the load direction, since in a real composite, once the brittle cementitious matrix cracks, the fibers are not only embedded to various depths on both sides of the matrix but also inclined at various angles with respect to the loading direction. Further, fibers pulling out as a bundle have a very different performance as compared to a single fiber owing primarily to fiber-fiber interaction. Also, in a real composite, the contribution from the matrix is not entirely absent while fibers are pulling out (as assumed in an ideal pull-out test) due to aggregate interlocking, discontinuous cracking and crack bands. Thus, the idealistic single fiber pull-out test with the fiber aligned with respect to the loading direction is not a realistic representation of what is happening in a real composite. So far, no attempt has been made to rationally optimize the fiber geometry with respect to the properties of the matrix material, i.e. concrete, and the fiber material, i.e. steel or other metal.